$\approx$ In Problems $11-28$, sketch the region bounded by the graphs of the given equations, show a typical slice, approximate its area, set up an integral, and calculate the area of the region. Make an estimate of the area to confirm your answer.
$$ y=3-\frac{1}{3} x^{2}, y=0, \text { between } x=0 \text { and } x=3 $$